1. Field of the Invention
The present invention relates to a process for production of hot-finished tube of carbon, alloyed, and high-alloy steels in which a pretreated, degassed, and deoxidized liquid steel having the required chemical composition is continuously cast, separated into lengths, heated to a forming temperature, and formed in a pipe mill to any circular cross-section. More specifically, the present invention relates to a process for the production of tubing for antifriction bearings.
2. Description of the Related Art
Standardized hypereutectoid steels having a high carbon content are known, for example, by the DIN designation 100Cr6. This material is used to produce hot-finished tubes intended for use as the starting material for manufacturing antifriction bearing rings.
During conventional manufacturing of these tubes, an ingot is cast and rolled to form a tube round in a roughing mill. The tube round is then further processed to produce a hot-finished tube in a mill such, by way of example, as in an Assel mill. The Assel mill is located downstream of a rotary hearth furnace and generally includes a piercing unit comprising a rotary piercing mill for producing a hollow body which is then fed to three evenly distributed grooved peripheral piercer rolls and a bar serving as an internal tool. The hollow body is normally subjected to an intermediate heating and reduced in a multiple-stand reducing mill and a downstream rotary sizer to produce a hot-finished tube. A drawback of this process is that the size of the starting material, i.e., the tube round, must be close to that of the last finished table. Therefore, a large number of sizes of rolled and/or forged tube round materials are required to produce an entire product range of hot-finished tubes.
Other pipe mills such, for example, as push bench plants or mandrel mills, may also be employed instead of Assel mills for producing tubing for antifriction bearings. However, these other mills must always use preformed and homogenized charge material.
Instead of using an ingot, it has been suggested to use a continuously cast bloom that is rolled or forged after it has been cut off. The rolling or forging process must always be preceded by a diffusion annealing to break down or reduce segregations due to the casting process as well as coarse carbide segregations.
The above processes for producing starting materials require capital intensive forming equipment and are accordingly expensive. In addition, the large number of work and transportation stages required for these processes entail the risk of generating additional faults and/or intensifying existing ones. These faults must be eliminated at additional cost.
A search for further processes uncovered another prior art process for producing hot-finished tube made from a high-carbon steel such, for example, as hypereutectoid steel, disclosed in German reference DE 195 20 833 A1. This process offers a cost advantage over the other known processes and allows a better utilization of the material while reducing the processing times. The essence of this process includes the employment of unformed vertically cast strand or a strand cast in a bow-type continuous caster made from steel of any cross-section, particularly steel belonging to the material group of antifriction in a pipe mill without requiring the customary upstream rolling and forging processes of the homogenizing treatment. The elimination of these work stages produces a considerable savings in terms of both time and money. In addition, this process produces an improved utilization of materials because the material does not have to be separated and cropped as often.
After being separated into charge lengths, a continuously cast bloom produced in accordance with this process is heated to a forming temperature without preliminary forming and fed to a piercing press. The continuously cast bloom may also be fed to an elongator and a push bench after passing through the piercing press. A 2-roll cone-type piercer may be used preceding a mandrel mill or a plug rolling mill. It is known in the art that creating a tensile state with a minimized amount of tensile stresses in the workpiece eliminates the risk of having the workpiece burst during the piercing process. However, it is not known how to minimize the tensile stresses when a cone-type piercer is used.
Minimizing the tensile stresses would be of special importance in the 2-roll cross-rolling process because this process is characterized by a high degree of tensile stresses in the area of the billet core which may lead to bursting of the billet core when a solid billet is cross-rolled without an internal tool or with a maladjusted internal tool. This stress characteristic of the 2-roll cross-rolling as well as the resultant "reeling effect", which causes the core zone to break up and leads to internal surface defects of the rolled hollow billet during 2-roll cross-rolling of the unformed 100Cr6 cast strand, are described in literature and known to those skilled in the art.